Conventionally, resin panels have been used in various applications such as transport machines such as automobiles and aircraft, building materials, housings for electrical devices, sports, leisure and the like. A resin panel includes a core material covered with skin material sheets. There are a resin panel including a core material whose only one surface is covered with a skin material sheet, and a resin panel including a core material whose both surfaces are covered with skin material sheets. The resin panel including a core material whose only one surface is covered with a skin material sheet is used in the case in which the other surface is not visually recognized by a user for use in building materials for example, and thus it is unnecessary to cover the other surface of the core material with a skin material sheet. A resin panel including a core material whose both surfaces are covered with skin material sheets is also referred to as a sandwich panel. The resin panel has two skin material sheets, and a core material that intervenes between the two skin material sheets. That is, a fundamental form of a resin panel is a stacked structure of a skin material sheet, a core material and a skin material sheet.
Conventionally, a structure is known including a resin foam body, as a core material, into which a metal reinforcing material is inserted. For example, Japanese Laid-open Patent Publication No. 2010-174577 discloses a resin panel including, in a hollow part of a hollow double-walled structure, an interior material that unites a core material and a reinforcing material by fitting the reinforcing material with the core material of thermoplastic resin. Before fitted, the core material is molded into a shape that is approximately the same as that of the space in the hollow part. According to the Publication, with the resin panel, the interior material may be suitably positioned, and rattling or deformation of shrinkage due to molding may be prevented.